Because of excellence in impact resistance, abrasion resistance and chemical resistance in comparison with general purpose polyethylene, high molecular weight polyethylene having an intrinsic viscosity [.eta.] of at least 5.0 dl/g has heretofore been used as engineering plastics in various fields. It has also been investigated to form films or sheets from the high molecular weight polyethylene having such excellent properties as mentioned above.
However, because the high molecular weight polyethylene is markedly high in melt viscosity in comparison with the general-purpose polyethylene, it is difficult to form said high molecular weight polyethylene by extrusion into film or sheet in such a manner as in the case of the general-purpose polyethylene. On that account, when a film or sheet is formed from the high molecular weight polyethylene, said high molecular weight polyethylene is first extruded into a rod, and the rod is then skived to obtain a film-like product. This process is still employed and practiced by some people, however, it is broadly true at present that almost all the films or sheets of the high molecular weight polyethylene are formed by the compression molding technique. In this connection, a film-like product obtained by skiving a rod extruded from the high molecular weight polyethylene has its strength of about 0.05 GPa, and this film is dense and non-permeable.
Although the films or sheets formed from the high molecular weight polyethylene by the above-mentioned process such as the compression molding technique are relatively high in impact resistance as well as in abrasion resistance, it is difficult to raise the production rate, and no molded products high in tensile strength are obtainable.
Accordingly, it is a common practice for those skilled in the art to prepare films, sheets or fibers excellent in strength from the high molecular weight polyethylene by molding a mixture comprising the high molecular weight polyethylene and a solvent or plasticizer capable of forming a homogeneous mixture together with said polyethylene into a film or the like.
For example, Japanese Patent L-O-P Publn. No. 177035/1982 proposes a process in which high molecular weight polyethylene having a molecular weight of more than one million is incorporated with a low molecular weight hydrocarbon compound to prepare a composition, and from this composition a sheet is formed. In this process, however, the sheet obtained has a tensile strength of about 0.035 GPa.
In this connection, 1 GPa may be converted into about 10,000 kg/cm.sup.2.
Furthermore, there are various known processes, for example, a process for obtaining biaxially oriented films of high molecular weight polyethylene by biaxial orientation of a mixture comprising a high molecular weight polyethylene and a solvent or plasticizer capable of forming a homogeneous mixture with said polyethylene, a process for obtaining microporous films by extraction and removal of the solvent or plasticizer contained in the above-mentioned biaxially oriented films of high molecular weight polyethylene, and a process for obtaining biaxially oriented films of high molecular weight polyethylene, wherein a sheet is formed from a mixture comprising a high molecular weight polyethylene and a solvent or plasticizer capable of forming a homogeneous mixture with said polyethylene, the solvent or plasticizer is extracted and removed from said sheet, and this sheet is then biaxially stretched.
For instance, Japanese Patent L-O-P Publn. No. 227420/1984 discloses a process for preparing biaxially oriented films of high molecular weight polyethylene, characterized in that a mixture comprising a high molecular weight polyethylene having an intrinsic viscosity [.eta.] of not less than 5 dl/g and a hydrocarbon plasticizer having a boiling point exceeding the melting point of said polyethylene is biaxially stretched more than 3 times in the lengthwise direction and more than 3 times in the crosswise direction at a temperature in the range of from not less than 60.degree. C. to less than the melting point of said polyethylene. This publication also teaches that axially oriented microporous films of high molecular weight polyethylene are obtained when the hydrocarbon plasticizer is extracted and removed by means of a suitable solvent from the above-mentioned biaxially oriented films of high molecular weight polyethylene.
Japanese Patent L-O-P Publn. No. 84224/1986 discloses a process for obtaining high molecular weight polyethylene films, wherein a gel-like product is prepared by incorporating polyethylene having a weight average molecular weight of at least 400,000 with a relatively volatile solvent which is liquid at room temperature, and this gel-like product is formed into a sheet, followed by biaxial orientation carried out at a temperature of 75.degree. C. or higher in the lengthwise and crosswise directions so that the draw ratio in each direction becomes 3 or more. The high molecular weight polyethylene films obtained by this process, though being smooth and lustrous, have the maximum opacity of 15%, the maximum water vapor transmission of 0.6, and are not porous.
Japanese Patent L-O-P Publn. No. 39602/1988 discloses a process for the preparation of high molecular weight polyethylene films, characterized in that a gel-like product is prepared by mixing polyethylene having a molecular weight of 500,000 or higher with a nonvolatile solvent such as paraffin oil, the gel-like product is formed into a sheet after adjustment of the amount of the solvent present in said gel-like product to 80-95%, and the sheet is stretched at a temperature below 120.degree. C. two times or more in monoaxial direction and biaxially stretched 10 times or more at an areal magnification, followed by removal of the solvent.
Japanese Patent L-O-P Publn. No. 255415/1985 describes high molecular weight polyethylene films composed of polyethylene having a viscosity-average molecular weight of 400,000 or higher and having a tensile strength of 0.25 GPa in every direction. This high molecular weight polyethylene film is prepared as a non-porous high-strength film by first forming a film from a mixture comprising a high molecular weight polyethylene and an aliphatic or alicyclic compound excellent in compatibility with said polyethylene, capable of being uniformly mixed therewith in a molten state, and capable of swelling or wetting the polyethylene, extracting said aliphatic or alicyclic compound from said film, and stretching the thus treated film at a temperature of 130.degree.-150.degree. C. and a stretch rate of 50%/sec. In this high molecular weight polyethylene film, it is designed that a decrease in mechanical strength as a non-porous high strength-film is prevented by stretching said polyethylene film at a specific stretch rate under application of heat as defined above, because when the aliphatic or alicyclic compound is extracted from the high molecular weight polyethylene film, the resulting film becomes a porous film, decreasing in strength.
The high molecular weight polyethylene films obtained by the processes as mentioned above., however, involved such problems that they are apt to undergo laminar separation, crease easily and poor in retention of shape thereof. Further, there were such problems that because of being poor in surface smoothness, the conventional biaxially oriented films of high molecular weight polyethylene sometimes cause blocking between them, stick to or twine round the machine parts at the contact portions thereof in the course of film forming operation.
In consideration of such prior art as mentioned above, the present inventors conducted extensive researches and, as the result, they have eventually found that biaxially oriented films of high molecular weight polyethylene excellent in surface smoothness and tensile strength and having gas permeability are obtained when a raw fabric sheet is formed from a blend of a high molecular weight polyethylene and a solvent or plasticizer, the solvent or plasticizer is removed from the raw sheet, and further the thus treated raw sheet is biaxially stretched to obtain a film, followed by heating under specific conditions. The present invention has been accomplished on the basis of the above findings.
Furthermore, an excellent affinity between the film surface of said biaxially oriented film and water is required according to the purpose for which this film is used. Surface of the biaxially oriented films of high molecular weight polyethylene may be hydrophylized in this case.